(Requires Appendix Material): Show That the AR(1)process Y_t = A₁Y_t-1 $\left| a _ { 1 } \right|$

(Requires Appendix material): Show that the AR(1)process Y_t = a₁Y_t-1 + e_t; $\left| a _ { 1 } \right|$ < 1, can be converted to a MA(∞)process.